Prepreg composites, such as graphite-epoxy have been molded into all sorts of lightweight structures, particularly in the aircraft industry. Typically, a number of plies are laid up into a female mold. However, application by this technique inherently produces composite parts with an oversized thickness in the female radii. Caul plates and even match metal tooling do not alleviate the problem.
Because of the variation of thickness of each ply tolerance, the male radii on the caul plate for matched metal tooling can give an outof-tolerance female radius. Accordingly, it is difficult to obtain high precision molding with composites.
The present invention utilizes an inflatable stiffened bladder which dimensionally matches the inside mold line. The bladder basically includes a resilient surface which presses a laid-up composite securely within a female mold form. A stiffened section of the bladder accommodates vacuum compaction.
Prior to insertion within the female mold, the individual plies of the composite are laid up onto an inverted bladder which serves as a male mold. These plies are periodically compacted before inversion and insertion into the female mold. As a consequence, the plies are maintained tightly against the male radius of the stiffened bladder; and any deviation of ply thickness is taken up by the inflatable bladder when it is inflated.
Inflation of the bladder occurs when a vent tube is subjected to autoclave pressure. Thus, the composite being molded can be cured in an autoclave while the bladder maintains its inflated state. This ensures, during the curing-molding interval, that the outer surfaces of the composite will be forced against the respective molding surfaces of the female mold and the contacting bladder. As a result, composites may be fabricated to exact replication of the molding surfaces